1. Technical Field
This invention is directed to spark plugs and other ignition devices used in internal combustion engines and, more particularly, to ignition devices having high performance metal firing tips.
2. Related Art
Spark plugs are well known in the industry and have long been used to initiate combustion in internal combustion engines. In general, a spark plug is a device that extends into a combustion chamber of an internal combustion engine and enables a spark to ignite a combustible mixture of air and fuel therein. Specifically, a spark plug typically includes a cylindrical metal shell having external threads that screw into a portion of the engine and further having a hook shaped ground electrode attached thereto at a firing end of the spark plug. A cylindrical insulator is disposed partially within the metal shell and extends axially beyond the metal shell toward a firing end and also toward a terminal end. A conductive terminal is disposed within the cylindrical insulator at the terminal end of the spark plug, opposite the firing end. At the firing end, a center electrode is disposed within the insulator and projects axially out of the insulator toward the ground electrode, whereby a spark plug gap is defined between the center electrode and the ground electrode.
Due to the very nature of an internal combustion engine, spark plugs are exposed to many extremes occurring within the engine cylinder, including high temperatures and various corrosive combustion gases, which have traditionally reduced the longevity of the spark plug. Spark erosion also reduces the longevity of spark plugs. Spark erosion is where the electrode and in particular the firing tip or a material next to or adjacent to the firing tip erodes away during operation due to localized vaporization due to arc temperatures. Spark plugs traditionally have electrodes formed from Nickel or Nickel alloys which are susceptible to spark erosion. Recently manufacturers have been forming the firing end of the center electrode out of a precious metal such as Platinum, Iridium, or alloys thereof to minimize spark erosion. Platinum, Iridium, and alloys thereof are typically very resistant to spark erosion. However, Platinum, Iridium, and alloys thereof are generally very expensive and it is desirable to minimize the amount of material used to provide the spark portion.
In operation, ignition voltage pulses of up to 40,000 volts are applied through the spark plug to the center electrode, thereby causing the spark to jump the gap between the center and ground electrodes. The spark ignites the air and fuel mixture within the combustion chamber or cylinder to create high temperature combustion to power the engine. Unfortunately, the high voltage and high temperature environment within the combustion chamber can degrade the components of the spark plug, such as through spark erosion. As the spark plug becomes degraded, the characteristic of the spark may become altered thereby degrading the quality of the spark and resulting combustion. While Platinum, Iridium, or other precious metals and alloys thereof are less susceptible to spark erosion, if too small of a piece, either in length, width, or size is used for the precious metal firing tip, the spark may jump around the precious metal tip and arc between the base material of the center electrode and the ground electrode. As the base material is typically a Nickel alloy, it is susceptible to spark erosion which may cause the base material or center electrode to erode away until the precious metal tip falls off. Any degradation of the plug will affect the quality of the spark and any spark that does not originate from the spark surface on the spark portion but instead originates on the center electrode and passes around the precious metal firing tip will degrade the quality of the spark. The quality of the spark effects the ignition of the mixture of air and fuel (i.e., the combustion efficiency, combustion temperature, and combustion products) thus, the power output, fuel efficiency, performance of the engine, and the emissions produced by the combustion of the air and fuel mixture may be adversely affected. Due to the increasing emphasis on regulating emissions for motor vehicles, increasing fuel prices, and modem performance demands it is desirable to maintain a high quality spark for consistent engine performance and emission quality.
The longevity of the spark plug and thereby resistance of the spark plug to spark erosion is also important to manufacturers. Manufacturers are increasingly requiring longer service lifetimes from spark plugs such as 100,000 mile, 150,000 mile, and 175,000 mile service lifetimes. Many traditional Nickel spark plugs only have service lifetimes of 20,000 to 40,000 miles due to spark erosion and corrosion. Furthermore, many manufacturers are increasing the compression within an engine cylinder to provide a more fuel efficient engine. Any increase in compression also requires an increase in operating voltage of the spark plug to sufficiently allow the spark to jump the spark gap between the center and ground electrodes. Any increase in the operating voltage of a spark plug also increases the likelihood of spark erosion and therefore reduces the longevity of the spark plug. One method to combat spark erosion is to significantly increase the amount of precious metal material such as Iridium, Platinum, or alloys thereof forming the tip spark portion or size of the firing tip. However, Iridium, Platinum, and alloys thereof are extremely expensive and as manufacturers continually demand cost reductions, it becomes important to minimize the amount of Iridium, Platinum, or alloys thereof used in spark plugs.
Furthermore, in manufacturing spark plugs having spark portions formed out of Iridium, Platinum, or alloys thereof, attachment of the spark portion to the center electrode base material may be difficult. The Iridium and Platinum alloys tend to be dissimilar in properties and are sometimes difficult to reliably weld to the base material of the center electrode. Additionally Iridium and its alloys are often very brittle causing difficulty in processing and attachment to the base material.